Practice Questions

While all levels contribute, the precise 3D shape of an antigen-binding pocket is a feature of the protein's tertiary structure. It is formed by the folding and precise juxtaposition of R-groups from different parts of a single polypeptide chain (in heavy and light chains).

Uracil is a pyrimidine base found in RNA. Like thymine (its counterpart in DNA), its structure is complementary to adenine, and it forms two hydrogen bonds with adenine during base pairing. Guanine pairs with cytosine.

Secondary structures are defined by the pattern of hydrogen bonds between the carbonyl oxygen (C=O) and the amide hydrogen (N-H) of the peptide backbone itself. The R-groups are not involved; their interactions define the higher-level tertiary structure.

Zymogens like pepsinogen, trypsinogen, and chymotrypsinogen are inactive precursors of powerful proteases. They are activated by cleavage only after reaching the gut lumen. This prevents them from hydrolyzing the proteins of the cells that produce them, which would lead to tissue destruction.

The active site catalytic residues often depend on specific ionization states to function. At the optimum pH, these residues have the correct charge (+ or -) for substrate binding or catalysis. Deviation from this pH alters the ionization, disrupting the interactions and decreasing activity.

Glycogen is more extensively branched than starch's amylopectin. Branching creates numerous terminal non-reducing ends. Glycogen phosphorylase can act on all these ends simultaneously, leading to a much faster release of glucose-1-phosphate to fuel the animal's high metabolic rate.

The alternating single and double bonds (conjugation) in carotenoids create a delocalized electron system that can absorb specific wavelengths of visible light. This makes them colored pigments (e.g., orange in carrots, red in tomatoes) that play roles in photosynthesis and photoprotection.

A nucleoside consists of a nitrogenous base plus a pentose sugar. A nucleotide is a nucleoside with one or more phosphate groups covalently bonded to the 5' carbon (or 3' carbon) of the sugar. The addition of phosphate is the defining difference.

"Amphipathic" describes a molecule with both hydrophilic (water-loving, polar head group) and hydrophobic (water-fearing, non-polar fatty acid tails) parts. This dual property forces them into a bilayer arrangement where the heads face water and the tails are sequestered away from it.

The classification depends on the carbonyl group's position. If the carbonyl is at the end of the carbon chain (C1), it is an aldehyde group and the sugar is an aldose. If the carbonyl is on an inner carbon (C2 for the most common ketose, fructose), it is a ketone group and the sugar is a ketose.